Conventional video systems require the use of many timing pulse controls to maintain proper operation and to display video data. Multiple pulse signals are required to trigger and time control operational functions of the video display. Conventional systems require separate cables to transmit each pulse signal. Such pulse control signals include horizontal blanking, horizontal synchronization, horizontal drive, vertical blanking, vertical synchronization, and vertical drive.
Pulse signals which control these particular functions are subject to uncertainty in their time control. This uncertainty is caused by the rise time required for the pulse signal to respond. Such rise time delays tend to cause instability in control of the video picture. This instability is usually reflected in horizontal jitter of the video picture as viewed by the observer.
Due to present technological improvements present video systems are tending more towards raster scan techniques. One principle reason for this is that system memories are becoming less expensive. Less expensive memories means that higher resolution displays become more efficient, economical and attractive for use. Conventional television standards in the United States provide video systems with approximately 525 TV lines. From this, one can reasonably expect to obtain a maximum of 500.times.500 pixels of resolution. However, 1000.times.1000 picture elements or pixels is optimum. To increase resolution from 500.times.500 to 1000.times.1000 pixels means the quadrupling of memory and bandwidth. Memory no longer creates a problem, however, bandwidth imposes limitations due to the fact that wider bandwidth requirements translate into rise time delay problems in control signal pulses.
The task of transmitting synchronization pulses to different memory plane circuits that unload and serialize the memory data into a serial bit stream of video is a key problem in the art. The unloading and serialization must be accomplished within a narrowband of time because of the wide bandwidth of the video.
Consider a typical case where the video bandwidth is 54 MHz which translates into an 18 nsec. bit, i.e., the time of one picture element or pixel. To maintain any control over the time stability of unloading and serialization, the jitter in the synchronization pulses must be significantly less than the pixel time. Therefore, the synchronization pulses must be stable over their period within technically two to four nsec.
To meet such a tolerance in timing, conventional synchronous generators must be of special design. However problems will still exist. The transmission of conventional synchronization pulses down long cables poses level detection problems in the analog comparators receiving the pulses. The detection problem manifests itself in jitter. Specifically the capacitance in the cable integrates the synchronization pulses so that the threshold of detection does not remain at a constant level.
Synchronization pulses from a specially designed conventional synchronization generator can be transmitted to the receiver by direct line connection or by other methods, such as Manchester encoding the transmitted signals. From a specially designed conventional synchronous generator, output pulses of typically two nsec rise and fall times can be Manchester encoded and transmitted differentially. The pulses thereby enjoy the same type tolerances in timing.
A disadvantage of this approach still exists. Each conventional synchronization pulse must have its individual Manchester encoder, cable, connectors, and Manchester decoder. Typically, a minimum set of synchronization pulses includes horizontal synchronization, horizontal drive, horizontal blanking, vertical synchronization, vertical drive, and vertical blanking. The extra hardware required by the system means more volume, weight, power, and cost. A need clearly exists to identify and present a system that overcomes these limitations and incorporates a method by which only one cable need be used to transmit synchronization signals to their receivers within the video system.
The invention presented herein is a digital raster timing encoder/decoder system which uses only one cable to transmit digitized signals that contain all necessary information of the conventional synchronization pulse signals as well as other desired information.